1. Field of the Invention
The present invention relates to the technical field of fingerprint recognition devices and, more particularly, to a two-substrate fingerprint recognition device.
2. Description of Related Art
Due to the fingerprint recognition being newly introduced in the application of the smart phones, the fingerprint recognition develops and becomes a popular issue again. As commonly known, there are capacitive fingerprint sensors and optical fingerprint sensors.
Fingerprint recognition sensors can be classified into semiconductor sensors and optical sensors according to their manufacturing processes. The semiconductor sensors are regarded as the fingerprint sensors produced by integrated circuits and semiconductor manufacturing processes. For capacitive semiconductor fingerprint sensors, the capacitance sensors are densely integrated in a chip. When a finger presses the surface of the chip, the capacitance sensors in the chip will produce a fingerprint image according to the amount of charge of the fingerprint, which is various from a peak to a valley of the fingerprint. The newly developed smart phones typically utilize the function of the capacitance sensors.
The capacitive semiconductor fingerprint sensors must have enough pressing area provided with sensing electrodes, which occupies a large area in a chip, resulting in that the production cost of the chip is greatly increased. On the other hand, the chip per se is a bare semiconductor chip. When designing the chip, it is a challenge to consider special package manufacturing processes for isolating the perspiration from the finger, avoiding the acid-base erosion on the surface of the chip, and protection for electrostatic discharge. When using a common protection glass to protect a bare semiconductor chip, since the thickness of the protection glass is about 200˜300 μm, the capacitive semiconductor fingerprint sensor chip under the protection glass is difficult to distinguish the small variation of the amount of charge from the peak to the valley of the fingerprint, and thus is difficult to effectively perform fingerprint sensing. To solve the problem, the newly developed smart phones attach a sapphire substrate with a high dielectric constant on the surface of the fingerprint sensor for protection. A sapphire substrate is hard enough, so that the thickness thereof can be reduced to be smaller than that of a protection glass, thereby solving the problem of the low efficiency of the conventional capacitance sensor. However, a sapphire substrate is not satisfactory to the users as it is much more expensive than a common protection glass. Thus, it is desired to provide an improved fingerprint recognition device so as to mitigate or alleviate the aforementioned problems.